Anthropogenic fingerprint detectable in Upper Tropospheric ozone trends retrieved from satellite

Tropospheric ozone (O-3) is a strong greenhouse gas, particularly in the upper troposphere (UT). Limited observations point to a continuous increase in UT O3 in recent decades, but the attribution of UT O3 changes is complicated by large internal climate variability. We show that the anthropogenic signal ("fingerprint") in the patterns of UT O-3 increases is distinguishable from the background noise of internal variability. The time-invariant fingerprint of human-caused UT O-3 changes is derived from a 16-member initial-condition ensemble performed with a chemistry-climate model (CESM2-WACCM6). The fingerprint is largest between 30 degrees S and 40 degrees N, especially near 30 degrees N. In contrast, the noise pattern in UT O-3 is mainly associated with the El Nino-Southern Oscillation (ENSO). The UT O-3 fingerprint pattern can be discerned with high confidence within only 13 years of the 2005 start of the OMI/MLS satellite record. Unlike the UT O-3 fingerprint, the lower tropospheric (LT) O-3 fingerprint varies significantly over time and space in response to large-scale changes in anthropogenic precursor emissions, with the highest signal-to-noise ratios near 40 degrees N in Asia and Europe. Our analysis reveals a significant human effect on Earth's atmospheric chemistry in the UT and indicates promise for identifying fingerprints of specific sources of ozone precursors.